1. Field of the Invention
The present invention relates to a wavelength selective switch.
2. Description of the Related Art
A wavelength selective switch controls an angle of a mirror in a mirror array according to a wavelength. In such a wavelength selective switch, while a light of a certain wavelength from an arbitrary input port is being output to an output port, a light from an unintended input port should not be output to an output port when an input port is switched to a different input port. As one of the methods to prevent the light of the unintended input port from being output to the output port, the mirror can be once panned in a light dispersion direction to sufficiently suppress an intensity of the light from the input port being coupled. In this method, the input port is switched to an arbitrary input port by panning the mirror again in the angle dispersion direction after the mirror is panned in a port switching direction under a state in which the light from any input port is not output to the output port (for example, U.S. Pat. No. 6,798,941). This method is generally referred to as hitless method.
Furthermore, it is widely known that broadening of a band of the wavelength selective switch can be effectively achieved by decreasing a spot diameter on the mirror in the mirror array.
However, if the spot diameter is decreased to achieve the broadening of the band, a decrease in a transmittance with respect to a panning angle of the mirror is reduced during the hitless operation, thus causing difficulty in executing the hitless operation. Accordingly, a panning amplitude of the mirror should be increased to achieve broadening of the band of the wavelength selective switch in the hitless operation; however, there has been a problem that the increase in the panning amplitude is limited by a structure of the mirror.